1. Field of Invention
The invention relates generally to Class-D audio amplifiers and more specifically to use of a front end PWM controller integrated circuit and a single source power output stage for Class-D amplifiers.
2. Description of the Related Art
A Class-D amplifier, also known as a switching amplifier, is an amplifier that switches at a high frequency, generating a high-voltage rectangular waveform at its output. This rectangular waveform can be modulated with a low-voltage signal within the audio bandwidth. The modulation results in a pulse width modulated (hereinafter referred to as "PWM") waveform at its output. Field effect transistor (FET) circuitry produces a high voltage PWM waveform, which is filtered with a passive inductor capacitor (LC) filter to remove the high-frequency carrier waveform and reconstruct the high-voltage, low-frequency waveform from the modulation input command. The filtered high-voltage waveform is now in the audio bandwidth and when applied to a speaker will produce sound.
Presently and in the past, developers implementing Class-D audio amplifiers have done so by using discrete circuitry such as op-amps, comparators, etc. Discrete circuitry has typically been considered essential to generation of a low distortion audio signal output.
FIG. 1 illustrates the Classic Half-Bridge Class-D audio amplifier 10 using discrete components to generate the PWM output. A triangle wave generator 12 creates a triangle waveform carrier frequency. The triangle waveform sets the resulting switching frequency of the amplifier and is typically greater than ten times the audio frequency bandwidth, 200 KHz. Conventional voltage comparator 16 compares the triangle waveform with the error signal to produce a PWM output signal. The error signal is formed by the error amp 18 when the command input signal (audio input) at 20 is compared with the actual signal taken from the output at junction 28. The control-loop response of the system determines how quickly the output can respond to the input command and produces the error signal. The error signal is proportional to the difference between the audio input signal and the actual signal at the output. The PWM signal from the comparator 16 is then sent to a field effect transistor (FET) driver integrated circuit 19 that drives the output FET's 20, 22. The FET's consist of an upper P-channel output FET 20 and a lower N-channel output FET 22. The P-channel FET switches a bus voltage supplied from +VIN and the N-channel FET switches a bus voltage supplied from -VIN to produce a high-voltage PWM waveform that is filtered by the LC filter 24 to reproduce the audio signal at the speaker 26.
A half-bridge topology is shown in FIG. 1 for simplicity, however, this concept can be extended to a full-bridge topology using four N-channel FETs 30, 32, 34, and 36 as shown in FIG. 2. The full-bridge topology as shown in FIG. 2 requires a positive power supply. FIG. 2 shows a single power source +VIN. Full-bridge topology requires an increase in discrete components. In addition to the triangle wave generator 12, comparator 16, and error amp 18, full-bridge topology requires a differential amp 30, which requires more complex output signal processing for the feedback circuit. In operation, the differential amp 30 receives the differential signal and produces therefrom an output signal to combine with the audio input signal. The topology chosen is determined by the output power requirements of the amplifier. Higher power amplifiers, those greater than 200 watts, will typically use a full-bridge topology. An LC filter 24 is adapted to filter the differential output to reproduce the audio signal at speaker 26.
Present Class-D amplifier devices do not use integrated control chips having triangular (or sawtooth) waveform generators, comparators, and error amps to produce a PWM signal. Traditional Class-D amplifiers are described in the May 25, 1995 EDN publication entitled Class D Amplifiers Provide High Efficiency For Audio Systems, by Jeffrey D. Sherman, and is hereby incorporated by reference. Traditional Class-D amplifier technology uses discrete components to produce audio output signals having low total harmonic distortion (THD), low intermodulation distortion (IMD), and the like. Discrete components also provide control of over-current limiting, thumping during turn-on and turn-of, soft clipping, and further provides discrete operational amplifiers used to process feedback signals, and accomplish other basic signal processing functions. However, use of discrete operational amplifiers requires additional circuit space requirements, and add considerable cost to the design of the Class-D amplifier. In addition, as shown in FIG. 1, the half-bridge Class-D amplifier requires a dual power supply having an upper positive and lower negative power supplies. The positive power supply requires use of an expensive P-channel FET in the upper leg. As a result of the substantial amount of discrete components currently used in current Class-D amplifiers, the cost of Class-D amplifier designs have been prohibitive for high-volume consumer applications such as multimedia, PC-TV, home theater, automotive and general consumer applications.
It is therefore an object of this invention to provide a novel input structure in a Class D-Amplifier comprising an integrated controller circuit to simplify Class-D amplifier circuitry and to minimize the use of expensive discrete components.
It is an object of this invention to provide a novel output structure in a half-bridge Class-D amplifier including a single positive power supply, two N-Channel FET's, and a coupling capacitor placed prior to or after a LC filter.
It is an object of this invention to eliminate the expensive P-channel FET for the Half-Bridge Class-D Amplifier Design.
It is another object of this invention to provide a simplified and low cost Class-D amplifier for high-volume consumer applications.